Structural ceramics applications typically involve the supporting of mechanical stresses (e.g., from structural loads), the endurance of thermal stresses, and/or resistance to wearing conditions. In this area, sintered silicon nitride ceramics are very promising because of their potentially excellent mechanical properties.
Unfortunately, most known sintered silicon nitride structural ceramics are made from very expensive high grade silicon nitride powders (typically $25-60/lb.). Consequently, the high cost of the sintered product is a major limiting factor on the use of silicon nitride ceramics.
High grade powders typically contain very low total amounts of impurities (i.e., typically less than 2.0 wt. %). Impurities such as Fe, oxygen, or free silicon are each typically present at less than 0.1 wt. %. Indeed, methods of forming sintered silicon nitride ceramics for structural applications often specify that impurities be kept to a minimum to avoid degradation of mechanical properties.
High grade Si.sub.3 N.sub.4 powders also usually contain a high proportion (at least 85-95 wt. % of the total Si.sub.3 N.sub.4) of alpha phase silicon nitride which is the preferred starting phase for forming sintered silicon nitride bodies. The high grade powders correspondingly contain less than 15% beta phase silicon nitride. The most desirable high grade powders may not contain any beta phase at all. Large amounts of beta phase are avoided because it is generally thought that high beta phase powders are very difficult to sinter to high density.
Another characteristic of high grade powders is their fine particle size, typically in the submicron range. Fine particle size also generally facilitates obtaining a good sintered structural ceramic product.
Most silicon nitride structural ceramics are sintered at relatively high temperatures (i.e., at least about 1800.degree. C.). The requirement of high sintering temperatures is another factor which increases the cost and decreases the applicability of known silicon nitride structural ceramics.
Requirement of high grade starting powders and high sintering temperatures has made structural silicon nitride ceramics quite expensive. Correspondingly, the use of silicon nitride structural ceramics has been limited by cost of the product. Consequently, there is a great need for less expensive silicon nitride ceramics which still provide good mechanical properties for many structural applications.
Refractory grade and other low grade silicon nitride powders are much less expensive than the high grade silicon nitride powders used to make structural ceramics. These silicon nitride powders are principally used for high temperature non-structural applications, such as for furnace linings. However, refractory grade and other low grade silicon nitride powders are generally thought to be unsuitable for producing sintered silicon nitride structural ceramics.
Refractory grade and other low grade silicon nitride powders usually contain relatively high amounts of impurities such as iron, oxygen and free silicon. These impurities often form relatively low temperature liquid phases that can be detrimental to the high temperature strength of a sintered product.
Refractory grade and other low grade silicon nitride powders also often contain high proportions of beta phase silicon nitride. Beta silicon nitride in the starting powder is generally thought to be very detrimental to obtaining a sintered product of desirably high density.
Refractory grade and other low grade silicon nitride powders also have a coarser particle size which is also less conducive to obtaining a sintered product of high density. Since these powders are used in furnace linings and the like, there is no need to incur the additional cost associated with particle size reduction or the synthesis techniques that produce lower initial particle sizes.
These characteristics of refractory grade and other low grade silicon nitride powders have discouraged use of such powders as starting materials for producing sintered silicon nitride structural ceramics. Thus, there remains a need for an inexpensive silicon nitride structural ceramic material.